In the above optoelectronic unit used in optical communication and optical information systems, an optical isolator is required which has the function that any reflected light produced in a transmission system does not return to the semiconductor element side when the light emitted from the semiconductor element such as a semiconductor laser element is made to enter a transmission system constituted of an optical fiber, i.e., the function to prevent the semiconductor element from operating unstably because of reflection return light.
The semiconductor element is also required to be perfectly isolated from the air outside in order to prevent its function from lowering because of the atmosphere or the atmospheric humidity.
Moreover, the semiconductor element may change in characteristics with its temperature variations. Hence, its temperature is commonly controlled by a Peltier device. However, since the optical isolator also has temperature characteristics, the temperature is controlled by the Peltier device also in respect of the optical isolator in the case of a semiconductor module constituting a high-performance optoelectronic unit.
Meanwhile, the cost of assembling of such a semiconductor module constituting an optoelectronic unit can be made lower as the component parts for assembly are in a smaller number. However, any conventional optical isolator is set independent from optical-fiber component parts, and hence, in order to put these into an assembly, it has been necessary to provide an assembly equipment for exclusive use.
Accordingly, in order to achieve low cost for semiconductor modules, as shown in FIG. 5, an optical isolator module g has been proposed in which an optical isolator c and an optical fiber a are set integral (Japanese Patent Application Laid-open No. 6-194548). In using such an optical isolator module g, however, because of the structure of this optical isolator module g, the temperature of the whole housing i of a semiconductor module h fitted with the optical isolator module g must be controlled, and hence it has been difficult to control the temperature of the optical isolator c. Incidentally, in FIG. 5, letter symbol b denotes a capillary to which the optical fiber a has been fitted; d, a transparent member; e, a semiconductor laser element; and f, a focusing lens n.
In a semiconductor module required to have an especially high reliability, it has also been necessary in some cases to provide a structure in which any component parts making use of a resin are completely isolated from the semiconductor element so that its function can be prevented from lowering because of any gases released from an adhesive used for bonding optical component parts.
Accordingly, in the conventional optical isolator modules, it has often been observed to employ a method in which the optical component parts are assembled by fastening them with solder without use of any adhesive.
Then, where the semiconductor element set in the semiconductor module is a semiconductor laser element, its emergent light is linear polarization, and hence an isolator of a polarization-dependent type can be used. In this case, a method can be employed in which a metallic film is added to the side of each of a glass polarizer and a Faraday rotator and this is fastened to a substrate material with solder. There, however, has been a problem that an attempt to replace the glass polarizer, which is expensive, with a relatively inexpensive birefringent crystal, e.g., a crystal material such as rutile, lithium niobate or YVO4 for the purpose of cost reduction makes it very difficult to fasten the metallic film with solder because these materials have a great anisotropy in the coefficient of thermal expansion and also the materials themselves are brittle.
Meanwhile, where the semiconductor element set in the semiconductor module is a polarization-independent typea semiconductor amplifier, it is essential to use a polarization-independent type optical isolator making use of a birefringent crystal. In fact, however, any solder-fastenable polarization-independent type optical isolator has not been put into practical use.
The present invention was made taking note of such problems. Accordingly, objects of the present invention are i) to provide an optical isolator module which has a structure making it easy to hermetically seal the semiconductor module and makes it possible that optical component parts made up using an organic material such as an adhesive are made airtight in the housing and also that the semiconductor element and the optical isolator is simply and easily temperature-controlled, and particularly ii) to materialize a structure applicable in the polarization-independent type optical isolator making use of a birefringent crystal, which is essential when the semiconductor module is the polarization-independent type semiconductor amplifier module, to enable manufacture of an inexpensive semiconductor amplifier module.